When metering small amounts of low boiling liquified gas, especially with a liquid nitrogen metering device, an even throughput of the liquified gas which can be shut off at any time and which can, on demand, be turned on and off in a cyclic manner must be attainable.
DE-OS No. 27 32 318 discloses an apparatus for metering liquid nitrogen whereby a disk which rotates about a vertical axis is moved along under the orifice for liquid nitrogen. Depending upon the shape of the disk, the orifice is closed to a greater or lesser degree. Only a cyclic metering of the liquid nitrogen is possible with such a mechanical apparatus.
It is furthermore known from DE-OS No. 31 41 465 that one can regulate the metering of the low boiling liquified gas through a needle valve which forms the orifice. The valve stem of the needle valve is hereby pulled upward so that liquid nitrogen can exit from the body of the vessel of the metering device.
It is, however, necessary after prolonged idle periods with emptying vessel and subsequent refilling, to make operational the valve opening which is either frozen shut or which can no longer close tightly by means of a heating arrangement. Also, ice crystals which occur during operation due to moisture in the air lead to an uneven flow of the liquified gas.